The present invention relates to a display tube and particularly, but not exclusively, to a large screen area, typically 0.75 to 1 M.sup.2, flat panel display tube employing cathodoluminescence.
Problems with large screen area display tubes of the conventional design are their large depth, large weight and high power consumption. One attempt to resolve these problems is disclosed in an article entitled "Large-screen flat-panel television": A new approach, by T.L. Credelle in R.C.A. Engineer, 26-7, July/August 1981, pages 75 to 81, and in British Patent Specification No. 2,005,070A. In order to reduce weight, Credelle divides the interior of the tube envelope into a plurality of horizontally adjacent modules using vertical support walls which contact the inside of the faceplate or front wall and provide a support for the faceplate glass. In consequence the faceplate can be of a thickness, typically 6 mm, which is substantially thinner than for a conventional cathode ray tube faceplate--such as a 25 inch (62.5 cm) kinescope faceplate which is approximately 12 mm thick. As far as power consumption is concerned, Credelle's goal is to limit it to four times that of a conventional tube having a quarter of the screen area.
In the modular tube mentioned above, each module has means for producing three high current, low voltage electron beams which are directed vertically upwards along paths which are parallel to the rear wall of the rectangular, flat panel envelope. Because of the possibility of the beam blowing-up due to space charge effects and because of the need to deflect the electron beams forward towards the screen, a ladder beam guide is provided adjacent to, but spaced from, the rear wall, and additionally vertically spaced-apart, horizontally elongate electrodes are provided on the rear wall; there being one electrode for each space in the ladder beam guide. The ladder beam guide serves to refocus the electron beams at intervals corresponding to every one or two picture elements in the vertical direction to prevent them blowing-up and, in conjunction with the horizontally elongate electrodes, deflects the beams from their vertical paths in the frame direction. Substantially planar, apertured focussing and accelerating grids are arranged parallel to the ladder beam guide to focus and accelerate the deflected beam towards a shadow mask positioned in front of the screen. Coverging and line scanning electrodes are provided on the support walls defining the lateral boundaries of the module to converge the beams on the shadow mask whilst it undergoes line scanning.
A disadvantage of this display tube is the need for high current, low voltage electron beams, because there is no provision to amplify the beam current. Consequently, means such as case the ladder beam guide must be provided to stop the beams from blowing-up. The ladder beam guide comprises a mechanically fragile, precision made mesh-like stucture which is expensive to make because of the close tolerances required to maintain beam focus.